1. Field of the Invention
The present invention relates to a spectrum diffusion communication receiving apparatus for receiving a signal diffused by a diffusion code.
2. Related Background Art
In a spectrum diffusion communication system using a direct diffusion method, original data such as a voice or data is converted into a base band signal having a band width much larger than that of the original data by using a diffusion code sequence such as a pseudo noise code. The base band signal is further converted into an RF (radio frequency) signal by coding schemes such as PSK (phase shift keying) and FSK (frequency shift keying). At a receiving side, reverse diffusion for obtaining a correlation with respect to a received signal is performed by using the same diffusion code as in a transmitting side, thereby converting the received signal into a narrow band signal having a band width corresponding to the original data. Subsequently, normal data decoding is performed to regenerate the original data.
In a spectrum diffusion communication receiving apparatus, the same diffusion code as in the transmitting side must be used to obtain the correlation as described above. Therefore, the diffusion code sequence included in the received signal must be synchronized with a decoding diffusion code sequence in the receiving side.
In the conventional spectrum diffusion communication receiving apparatus, code synchronization is realized by using a sliding correlation loop as shown in FIG. 20.
Referring to FIG. 20, a received diffusion signal is multiplied by a reference diffusion code sequence generated from a diffusion code generating circuit 36 in a mixer 31. An output from the mixer 31 is input to a band-pass filter (BPF) 32 having a band width corresponding to original data. An output from the BPF 32 is subjected to envelope detection by a detection circuit 33 and smoothed by a low-pass filter (LPF) 34.
When code synchronization is obtained between the diffusion code sequence included in the received signal and the reference diffusion code sequence generated from the diffusion code generating circuit 36 at the receiving side, a reverse-diffusion signal is obtained as the output from the mixer 31. Subsequently, the signal is transmitted through the BPF 32, subjected to envelope detection by the detection circuit 33, and smoothed by the LPF 34, thereby obtaining a high DC level.
When code synchronization is not obtained, no reverse-diffusion signal is obtained as the output from the mixer 31, and most of the received diffusion signal power is blocked by the BPF 32. Therefore, a DC level of a signal subjected to envelope detection and smoothed by the LPF 34 is much lower than that obtained when code synchronization is performed.
The DC level output from the LPF 34 is supplied to a voltage-controlled oscillator (VCO) 35. When code synchronization is not obtained, a DC level voltage of the output from the LPF 34 is sufficiently reduced. When the VCO receives a sufficiently low DC level voltage, it generates an output having a frequency slightly different from that of a diffusion code sequence included in the received diffusion signal. This output is supplied as a clock to the diffusion code generating circuit 36. When a clock rate of a reference diffusion code sequence generated in the diffusion code generating circuit 36 is slightly different from that of the diffusion code sequence included in the received diffusion signal, phases of the two sequences are gradually shifted from each other. As a result, code synchronization is obtained before the two phases are shifted by one period of the diffusion code sequence. A DC output voltage from the LPF 34 rises to lock the oscillation frequency of the VCO 35 at a current frequency, thereby obtaining synchronization between the received diffusion signal and the reference diffusion code sequence generated in the diffusion code generating circuit 36.
A sync determining circuit 37 normally monitors the output voltage from the LPF 34 and outputs phase information of the code of the diffusion code generating circuit 36 as a sync signal when the output voltage exceeds an arbitrary threshold level.
In the above conventional system, however, the phase of the reference diffusion code sequence must be shifted with respect to the received diffusion signal until code synchronization is obtained after a periodic operation is started. Therefore, a time required to obtain code synchronization is generally very long.
In another conventional system, three such sliding correlation loops are provided, and a diffusion code generated by a common diffusion code generating circuit is phase-shifted by an arbitrary amount to obtain phase-shifted diffusion codes (e.g., codes having a reference phase, a 1/2-advanced phase, and a 1/2-delayed phase). These phase shifted diffusion codes are input to three correlation circuits to triple a correlation detection period.
In this conventional system, however, a circuit for shifting the phase of a diffusion code is required, and three correlation circuits must be used.
In addition, since three band-pass filters and three envelope detectors are also required, a circuit arrangement and therefore adjustment are complicated.